Blast furnaces are generally fed with hot gas received from a heat regenerator such as a hot stove or a pebble heater. One inherent feature of such regenerators is the fact that the temperature of the hot gas leaving a regenerator during the blast stage gradually decreases. Also, it is generally desired to feed hot gas of constant temperature to the blast furnace. The gradually decreasing temperature of the hot gas therefore has to be adapted by mixing in cold gas in amounts chosen to obtain a constant temperature. Indeed, the temperature variations of the hot gas must be leveled out. This is generally achieved by mixing in cold gas in amounts sufficient to lower the temperature of the hot gas to its lowest value, i.e. to the temperature of the hot gas at the end of the blowing cycle. By adding the correct amount of cold gas to the hot gas, the temperature of the hot gas is lowered to a constant temperature in order to feed a gas at constant temperature to the blast furnace.
In recent years, it has been found that it may be beneficial to feed two separate streams of hot gas to the blast furnace at two separate levels to improve the efficiency of the blast furnace. Preferably, the two separate streams of hot gas are furthermore at different constant temperatures. This has e.g. been suggested in co-pending patent application LU 91 542 filed on Mar. 17, 2009.